There are movies you’re sure will never get made. Hollywood's adaptation of “Akira.”A fourth “Friday.” Another stab at Frank Herbert’s unfilmable “Dune” series. (David Lynch’s infamous 1984 version is impenetrable but awesome in its Lynchianweirdness.) But sometimes one of these somehow gets made anyway. Such is the case with “The Dark Tower,” the long-threatened movie take on Stephen King’s seven-volume “magnum opus.” It’s spent a decade in development hell, so long it seemed like it would never happen. And yet there it is on the summer calendar, holding up the Aug. 4 slot. And here’s a new trailer to prove it really is a movie that exists.

Explaining exactly what “The Dark Tower” is, however, is a trickier task. Part fantasy, part sci-fi, part-horror, part-Western, King’s series is a dense and, given its prolific author, predictably huge beast. Barring any future installments, it runs about 35 words longer than Marcel Proust’s entire “In Search of Lost Time.” (Someone make that into a movie franchise.)

The trailer spends a full three minutes trying to break it down for us: that there’s a parallel universe called “The Mid-World,” which looks like a Western but is actually semi-futuristic. We meet Idris Elba’s Roland, a gunslinger who’s searching for “The Man in Black” — a devious baddie played by Matthew McConaughey, who Roland assures us is “worse than the devil.” McConaughey wants to destroy a huge tower, which apparently separates this world from ours. If felled, it would usher in the apocalypse — or the kind of city destruction typically seen at the end of Marvel movies.

“The Dark Tower” series doesn’t have the same cultural cachet as King’s countless horror tomes. It’s for nerds by a nerd — a passion project King returns to every handful of years when he’s sick of pounding out epic, forest-killing scare-a-thons that get turned into terrible movies. Indeed, it’s important to remember the batting average for King movies is terribly low. How many of them are good? We’d say eight, maybe nine out of 40 or so, and the last terrific one was made all the way back in 1990: Rob Reiner’s “Misery.” (These things only work when they attract singular directors, like Brian De Palma, Stanley Kubrick, David Cronenberg and George A. Romero.)

But are the odds really against “The Dark Tower”? Even with McConaughey and Elba, it does look like a shadow of what it once could have been. When movie talk first began in 2007, it was to be a J.J. Abrams-Damon Lindelof production, provided they could take enough time off from “Lost.” (They couldn’t.) Then it was a Ron Howard project. Now it’s helmed by one Nikolaj Arcel, a Danish filmmaker whose last film was the saucy Alicia Vikander-Mads Mikkelsen costume drama “A Royal Affair.” His version incorporates some of the stuff Team Howard worked on during their failed stint, but it looks like they could only get it made after some cost-cutting. One thing that always got in the way was the huge budget required, but this one got it done at a comparatively skimpy $60 million. Whether that’s a bad thing is something we’ll find out in August.